Fix sort param in `search_fold` to guarantee unique values

Fixes #356.

The sort parameter must be type/bucket/key, as described in issue
duplicate values.  Other modifications include:

* Make sure to pass the new accumulator after calling `F` in
  `search_fold/9`.

* Increase the page size to 100 so less HTTP requests are made.  This
  should result in lower latency.

* Modify `yz_mapreduce` to use 1000 keys so since the page size was
  increased to 100.

* Add a test case to `yz_mapreduce` to verify that all 1000 unique
  results are returned.

* Add `yz_rt:load_module/2` to be used for loading a module's BEAM
  onto a set of nodes.  This is needed by the new test to have the HOF
  passed to `search_fold` available on the server process.